In the bottling industry, prior art plant for making plastic bottles comprises: a blow-moulding wheel mounted in such a way as to rotate about its longitudinal axis and equipped with at least one blow-moulding unit, which is fed by the blow-moulding wheel along a circular path extending around this axis and which comprises two half-moulds, each mobile relative to the other half-mould between a position of opening and a position of closing at least two cavities for blow-moulding bottles from respective parisons.
The two half-moulds are oriented in such a way that they are closed in a closing plane that is substantially radial with respect to the feed path of the moulding unit.
Prior art moulding units of this type have several drawbacks due mainly to the fact that inserting two parisons into a moulding unit with radial closing plane and then extracting two bottles from this type of moulding unit are relatively complex and difficult operations.
Also known in the prior art, from document DE102004057102, is a moulding unit where the half-moulds define a closing plane that is substantially tangent to the path followed by the moulds. In the solution described in that document, the half-mould on the outside is movable while the half-mould on the inside remains stationary (relative to the carousel it is associated with), that is to say, it does not move relative to the half-mould on the outside.
This solution, too, is not free of disadvantages, however, since the effectiveness and convenience of the closure at the bottom of the mould (that is, of the container moulding unit) are not entirely satisfactory. When the half-moulds are in the closed position, they form a hole at the bottom which is operatively closed by a base-mould.
As a result, the fact that one of the two half-moulds never moves relative to the other half-mould makes it difficult to move the base-mould between a mould closed position and a mould open position where the hole at the bottom is left free.
Moreover, the need to move the base-mould means that the form of the half-moulds and of the base-mould cannot be optimized in such a way as to close the mould hermetically, precisely because the base-mould must be moved away from the half-moulds without moving the inside half-mould. From patent document US2008/143022 it is known a method for controlling the opening and closing of a book-type blow mould for producing thermoplastic containers. The mould has two mould halves which can pivot through different angles. However, the closing plane of the moulds is radial with respect to the feed path of the moulding unit; therefore, this technical solution has the aforementioned drawbacks.
From patent document US2008/063742 it is known a blow moulding machine for the manufacture of containers based on performs. The moulds of said machine comprise a mould base which can slide vertically to open and close the bottom of the mould. However, also according to US2008/063742 the closing plane of the moulds is radial with respect to the feed path of the moulding unit.
Document WO2008/110887 of the same Applicant describes compression moulds for compression moulding of plastics in a pasty state, provided with means for heating said mould. Said mould comprises two half moulds, each having a corresponding bottom element with a tapered surface. However, this document does not regard blow moulding.